Timeline milestone
A second civilian GPS signal begins the modernisation era
From How GPS Became Invisible Infrastructure: Satellites, Smartphones and Precise Time
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Modernization
A second civilian GPS signal begins the modernisation era
The first GPS satellite carrying L2C launched in 2005, giving civilian users a second dedicated frequency and opening the way to better accuracy, reliability and ionospheric correction.
U.S. Air Force The launch of the first GPS IIR-M satellite introduced L2C, the system's second civilian signal. A receiver able to combine L2C with the original L1 civilian signal can compare the two frequencies to estimate and correct part of the delay introduced by Earth's ionosphere. L2C also improved acquisition and reliability for professional users. More importantly, it signalled a change in philosophy: civilian GPS was no longer a legacy by-product of a military system but an explicit target of long-term modernisation.
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